Technical Field
The present invention relates to an ion pump and a charged particle beam device using the same.
Background Art
Ion pumps are widely used as means for evacuating a device that requires a high degree of vacuum (i.e., a low pressure), such as an electron gun of a charged particle beam device (i.e., an electron microscope), to a vacuum. An ion pump can discharge not only common gas in the air, such as nitrogen, water, oxygen, carbon oxide, and hydrogen, but also noble gas such as argon and inert gas such as methane, and can obtain an extreme high vacuum of up to 10−8 Pa even when the ion pump is used alone. However, an ion pump has a characteristic that its hydrogen pumping speed decreases with decreasing pressure. As the major constituent of the residual gas in an extreme high vacuum range of less than or equal to 10−9 Pa is hydrogen, it is impossible to obtain an extreme high vacuum when an ion pump is used alone.
In recent years, a non-evaporative getter (hereinafter referred to as “NEG”) pump has been used to obtain an extreme high vacuum. A NEG pump is formed using a NEG alloy that is obtained by mixing a plurality of metals together. A NEG alloy is chemically active and has a high hydrogen pumping speed, but has a characteristic that it cannot discharge noble gas or inert gas. Thus, when a device is evacuated by using an ion pump and a NEG pump together, the drawbacks of the two pumps are compensated with each other, and an extreme high vacuum of less than or equal to 10−9 Pa can thus be obtained.
However, when the two pumps are mounted on a device, the device becomes complex and large, and also becomes expensive. Further, as the NEG pump is a pump that utilizes surface adsorption, it is necessary to periodically perform activation by heating the pump to a high temperature to maintain the pumping ability. Therefore, the maintainability of the device decreases.
Patent Document 1 below discloses an ion pump that continuously forms a NEG alloy on the surface of an anode by embedding the constituents of the NEG alloy in a flat-plate cathode of the ion pump and sputtering them. In particular, Patent Document 1 describes adjusting the weight ratio between zirconium and aluminum on the plane of the cathode 11, the sputtered area ratio, the sputtering rate, and a distribution of the ion density in accordance with the intended use, and consequently, 10−11 Pa can be obtained when the ion pump is used alone.